The present invention relates generally to the formation and development of charge patterns and more particularly to the formation of imagewise non-uniform charge patterns and the development thereof with finely divided marking material.
In conventional xerography, a photoconductive surface is uniformly charged in the dark with a charge of one polarity. The charged surface is exposed to a pattern of radiation to which it is sensitive, and the charge is dissipated in the radiation-struck areas. An imagewise uniform charge pattern remains in the non-radiation-struck areas.
The imagewise uniform charge pattern is normally developed by contacting the surface with a finely divided, colored toner which carries a charge of the opposite polarity. Because opposite polarities attract, the toner particles adhere to the photoconductive surface in the area of the uniform charge pattern.
The toner particles are most usually charged to the opposite polarity prior to development by rubbing contact with a carrier material. The carrier material is one which is removed from the toner material in the triboelectric series. The carrier material is usually in the form of particles of a larger size than the toner particles; although the carrier may, in some cases, be a liquid.
The toner is usually applied to the surface by cascading or flowing the toner or a toner-carrier combination (generally referred to as developer) across the surface. Other well known toner application methods include magnetic brush development, electrophoretic development and out-of-contact liquid development, such as that described in U.S. Pat. No. 3,084,043 to Gundlach.
Normal xerographic development has met with great commercial success. However, there remain areas where improvement is desirable. For example, photoconductive surfaces useful in most commercial xerographic development should be from about 10 to about 60 microns thick. Such thicknesses can be expensive and complicated to manufacture. Any imaging process which enables the use of thinner photoconductive layers would be an improvement.
The toner-carrier combination which is well known in normal xerography is somewhat dependent on the ambient relative humidity for successful operation. The humidity is preferably lower. Proper triboelectric charging of the toner is difficult if the humidity is too high.
Another difficulty of the toner-carrier combination is that the carrier can become coated with a thin layer of toner material after long periods of use. This is generally referred to as carrier aging. Such coated carrier material cannot be used efficiently to triboelectrically charge the toner material.
An imaging process which enables the use of a toner material which does not have to be charged to one polarity or another before development is also desirable. A toner material which is readily useful without a carrier material would also be an improvement.
In normal xerography, the toner particles adhere to (develop) the photoconductive surface at the point of charge differential. For example, in normal xerography a plate is charged to about 1,000 v. and then imagewise exposed. Exposure reduces the charge in the light struck areas to about 200 v., leaving about 800 v. in non-light struck areas. The line between a 200 v. area and an 800 v. area on a surface attracts toner particles (see FIG. 1). However, solid area coverage of a large area of uniform 800 v. charge cannot normally be accomplished without the aid of such sophisticated and complex mechanisms as magnetic brush developers or development electrode systems. U.S. Pat. No. 2,777,418 to Gundlach shows a typical development electrode used to achieve said area coverage of a large uniform charge pattern using charged toner. A development system which would make available solid area coverage without such complex mechanisms and with uncharged toner is desirable.
Even when magnetic brush development and a developer electrode are used to achieve solid area development, the problem of "developer starvation" is observed. This undesirable phenomenon manifests itself as a reduction of density as large solid areas are developed. The reduction can be quite dramatic and unattractive.
It is generally understood to occur because of the limited speed at which the typical toner-carrier type of developer can provide sufficient toner particles of the proper polarity. The development of a charge pattern by a toner particle of one charge leaves a net opposite charge on the carrier. This results in the carrier attracting the remaining toner with an increased attraction, making it more difficult for the remaining toner to leave the carrier and develop subsequent charge patterns. Normally this undesirable situation can be remedied only by replenishing the carrier with toner. A marking method which avoids developer starvation would be useful.
In normal xerography, a developed image must oftentimes be transferred to a receiving sheet if it is to be useful. Such transfer is a critical operation which must be handled carefully and with great control to achieve complete transfer while avoiding smearing.
There are normal xerographic methods which avoid transfer by coating a photoconductive layer on a conductive paper and developing the image directly on the coated paper. However, these methods require conductive papers and expensive coating treatments during manufacture. They also often lend themselves to liquid (bath) development which can be relatively slow and which sometimes can produce damp copies having an unpleasant odor.
An imaging system which can avoid the difficulties associated with the normal xerographic transfer step is desirable. An imaging system which enables development directly onto the final copy while avoiding the need for a photoconductive coating on the final copy and the need for liquid development also would be useful.
Other image development systems have been achieved which do not overcome these disadvantages. For example, in U.S. Pat. No. 3,318,698, F. A. Schwertz discloses a means for creating a charge pattern on an insulating surface. The surface is first frosted in an imagewise pattern and then uniformly charged. The frosted areas retain less of a charge than do the unfrosted areas, and a charge differential is created between the charged and uncharged areas. The charge differential of Schwertz is developable by well known xerographic methods. However, because the charge pattern of Schwertz is all of one polarity, it requires a toner which is triboelectrically charged to the opposite polarity. The system disclosed by Schwertz also has the solid area coverage problems discussed above in connection with charge differential development.
In U.S. Pat. No. 3,043,217 to L. E. Walkup, there is disclosed a development system in which a charge pattern is formed on an insulating surface and is developed with a typical xerographic developer. Although this system has many uses, it also shares many of the disadvantages of the Schwertz method.
In U.S. Pat. No. 3,250,636 to R. A. Wilferth, a magnetic imaging system is disclosed. In that system, a non-uniform pattern of magnetic microfields is established in a magnetizable layer. The uniform pattern is selectively removed by Curie point erasure, leaving an imagewise pattern of magnetic microfields. Curie point erasure is a well known technique and comprises heating a magnetized material above a known critical temperature at which its molecules become disoriented and the material loses its magnetic properties. Curie point erasure is sometimes accomplished by such techniques as flash heating a magnetized material with a Xenon flash lamp while protecting the image area with a mask.
While the technique of Wilferth avoids the solid area coverage problems of the prior art, it requires a magnetizable imaging layer and magnetically attractable toner particles and it is not compatible with well known optical imaging methods. Also, it is generally limited to forming dark images because magnetically attractable toners are most usually of a rust or black color.
Maksymiak, in U.S. Pat. No. 3,759,222 discloses the use of a non-uniform charge pattern on a transfer member to transport magnetic toner particles to an imaging member which carries a magnetic image. The transfer member of Maksymiak comprises a conductive drum coated with a thin dielectric layer on which is supported a conductive screen. A potential difference is established between the screen and the drum so that a non-uniform charge pattern exists over the surface of the drum. The transport member of Maksymiak does not provide an imagewise non-uniform charge pattern and does not overcome the difficulties of magnetic imaging pointed out above.
The existence of field gradients at the edges of xerographic charge patterns and in periodic xerographic charge patterns has been disclosed by H. E. J. Neugebauer (Appl. Opt. 3, 385 (1964) and R. M. Schaffert, Phot. Sci. Eng. 6, 197 (1962). However, the use of conductive toners to attempt to develop such field gradients results in discharge and loss of the image. The use of charged insulating toners or uncharged insulating toners results only in edge development, as described above and in connection with FIG. 1, below.